It appears that Quicktime X will have built in support for YouTube judging by a screenshot discovered by Apple Insider.

According to Apple Insider, "The application will let users take any supported video file and upload it directly to YouTube. Users will be prompted to enter their YouTube username and password, and QuickTime X Player will take care of the rest. This includes converting the movie into a file optimized for the video sharing service and then uploading it to the appropriate user account."